Figure 6.

Chk1 and Chk2 are apparently dispensable for p53- and Rb-dependent senescence pathways. (A) Growth curve of MEFs with or without RasV12-expressing mock (control), E7 (E7) or both E6 and E7 (E6/E7). The cells were infected with empty (open circles) or Ras-expressing retrovirus (filled circles). Cell numbers were counted at the indicated times after selection. Data are means±s.d. of triplicate experiments. Statistical significance of relative numbers of cells expressing RasV12 at day 8 compared with those expressing control vector was assessed by Student's t-test (^**P<0.01). (B) Morphology of cells obtained at 8 days after selection. The MEFs expressing mock (top panel), E7 (second and third panels) and E6/E7 (bottom panel) were infected with empty (top and second panels) or Ras-expressing retrovirus (third and bottom panels).